Fix failed tests in WPTestAttachments

[kurtpayne]

Uploaded files are not being removed between tests. When new files are uploaded in subsequent tests, their names are incremented by wp_unique_filename(), and this throws off the assertions.

This fix can be implemented in just WPTestAttachments::tearDown() but we should consider putting it in WPTestCase::tearDown() so it's effective for all tests.

Reset uploads in WPTestCase::tearDown

Perhaps just scandir() and unlink() will be enough. WP_Filesystem() could potentially trigger FTP stuff, we just want the direct methods.

Also, I think this is too heavy for WPTestCase::tearDown(). We should place it specifically in the WPTestAttachments::tearDown().

Patch attached for a recursive delete that does not rely on WP_Filesystem(). It's been targeted to just the tests that call wp_upload_bits().

Looks good. I think it would make sense to wrap this into $this->_destroy_attachments() or $this->destroy/empty_uploads(), similar to [675].

(In [677]) Introduce a _destroy_uploads() method for WPTestCase to wipe the uploads folder when appropriate. Additional tests may need to call this still. props kurtypayne, see #33.

(In [678]) Destroy the contents of the uploads folder in TestUploadFunctions?. fixes #33.

(In [679]) rmdir foo/ which is created in test_upload_dir_relative(). see #33.